services: Add 'virtual-terminal'.
Fixes <https://bugs.gnu.org/30505>.
Suggested by Danny Milosavljevic <dannym@scratchpost.org>.
* gnu/services/base.scm (unicode-start): Remove.
(virtual-terminal-service-type): New variable.
(console-font-shepherd-services): Remove 'modules'; remove call to
'unicode-start'. Add 'virtual-terminal' to 'requirement'.
(mingetty-shepherd-service, kmscon-service-type): Likewise.
(%base-services): Add 'virtual-terminal-service-type'.
* gnu/system/install.scm (%installation-services): Likewise.
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